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Metabolism of Other Hexoses

  1. Metabolism of Fructose:
    Essential fructosuria:  Defect in enzyme fructokinase (benign condition)
    Hereditary fructose intolerance: Genetic defect leading to phosphofructaldolase or Aldolase B deficiency leading to accumulation of fructose-1-phosphate in cell and consequently liver & kidney damage.

: Metabolism of fructose. Aldolase A is found in all tissues, whereas aldolase B is the predominant form in liver. (*, not found in liver.)


Hereditary Fructose Intolerance


Hereditary fructose intolerance is due to deficiency o/fructose 1, 6- biphosphate aldolase B in the liver, kidney and intestine.


Clinical Presentation

Aldolase B catalyzes fructose 1- PO4 to DHAP and D- glyceraldehyde.

This is severe disease of infants that appears with the ingestion of fructose containing food

Its deficiency causes excessive rise of fructose -1- PO4 and fructose in blood



Administration of fructose or sucrose leads to Hypoglycemia accompanied by nausea, vomiting, convulsion & profuse sweating.

Continued intake leads to



 Proximal renal tubular dysfunction

 Intellectual impairment

Treatment: Complete elimination of all sources of sucrose, fructose and sorbitol from the diet.

  1. Metabolism Of Galactose:
    Galactosemia condition is characterized by inability to metabolize dietary galactose due to deficiency of galactose-1-phosphate uridyl transferase.
    This results in increased level of galactose in blood and urine leading to cataract, mental disturbance, lethargy, vomiting, liver enlargement.


Fig:  Pathway of conversion of (A) galactose to glucose in the liver and (B) glucose to lactose in the lactating mammary gland. 

  1. Sorbitol Pathway
    Sorbitol pathway also known as polyol pathway, causes conversion of glucose to fructose via sorbitol. The pathway is greatly enhanced in uncontrolled diabetes mellitus, and is implicated in progression of chronic diabetic complication. It comprises of two sequential reactions, as shown below. 



Extra Edge
  1. The liver regulates the blood glucose after a meal because it contains the high-Km glucokinase that promotes increased hepatic utilization of glucose.
  2. Glucagon is secreted as a response to hypoglycemia and activates both glycogenolysis and gluconeogenesis in the liver, causing release of glucose into the blood.

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